Assuming you are in the uk, I would put it down to all the wet stuff we are getting.lol
Waterlogged roots can cause die-back so that's a distinct possibility. You could try giving them a good balanced feed in case they are short of some nutrient.
Or could it be earwigs eating the tenderest bit of the stem?
It`s definitely the weather conditions.
Are the in pots or in a boarder,Ive a couple in pots myself and alls fine with them ?.
Could you not lift the plants and put them in pots, John or are they to big.
Check the roots for Vine Weavil.